1. Field of the Invention
The present invention relates to an electronic unit for use with for example an IEEE 1394 serial bus, in particular, to a technology for transmitting a large amount of data using an asynchronous packet.
2. Description of the Related Art
A communication system that connects electronic units (hereinafter referred to as units) such as a personal computer, a digital video cassette recorder (hereinafter referred to as DVCR), and a digital television receiver with an IEEE 1394 serial bus and that sends/receives packets of a digital video signal, a digital audio signal, and a control signal therebetween has been proposed.
FIG. 1 shows an example of such a communication system. The communication system comprises a monitor 11, a DVCR 12, and a tuner 13 as units. The monitor 11 and the DVCR 12 are connected with an IEEE 1394 serial bus cable 14. The monitor 11 and the tuber 13 are connected with an IEEE 1394 serial bus cable 15.
In the communication system, an isochronous communication (referred to as ISO communication) for periodically transmitting real time data such as a digital video signal and a digital audio signal between units and an ASYNCHRONOUS communication (ASYNC communication) for non-periodically transmitting such as a unit operation control command and a unit connection control command can be performed. For example, a digital video signal and a digital audio signal selected by the tuner 13 can be reproduced as video information and audio information by the monitor 11. Alternatively, such signals can be recorded by the DVCR 12. In addition, a channel selection control command of the tuner 13, an operation mode setup command of the DVCR 12, and so forth can be sent from the monitor 11 to the relevant units through the IEEE 1394 serial bus cables 14 and 15.
In the communication system shown in FIG. 1, there is an AV/C (Audio Visual/Control) command set as commands for controlling AV (Audio Visual) units. In the AV/C command set, a status command for inquiring a status has been defined. In addition, as a response to the status command, status information of a designated unit that is sent back as an operand has been defined.
The data amount of the state may be very large. For example, as shown in FIG. 2, a television broadcast has a hierarchical structure composed of a network layer, a multiplex layer, a service layer, and a component layer. Thus, the data amount of a status command for inquiring each service (broadcast channel) that the digital broadcast tuner is currently selecting may exceed 30 bytes. In the digital broadcast, a plurality of services can be placed on one stream. Thus, a response to an inquiry is required for a plurality of services. Consequently, the data amount of one response may become several hundred bytes.
On the other hand, since the sizes of a command register (buffer) and a response register (buffer) of an FCP (Function Control Protocol) of the IEEE 1394 serial bus are up to 512 bytes, a command packet and a response packet whose sizes exceed 512 bytes cannot be transmitted and received. In addition, it is not assured that a real unit have a buffer that can store data of 512 bytes (the data amounts of currently available buffers are in the range from several ten bytes to one hundred and several ten bytes). When the buffer size is limited, information corresponding to an inquired state cannot be obtained.